Use stricter serializer for unions of simple types #4178

Job	Run time
build pgo-optimized on ${{ matrix.os }} / ${{ matrix.interpreter }}	0s
rust benchmarks	2m 50s
build on ${{ matrix.os }} (${{ matrix.target }} - ${{ matrix.interpreter \|\| 'all' }}${{ (((matrix.os == 'linux') && format(' - {0}', (((matrix.manylinux == 'auto') && 'manylinux') \|\| matrix.manylinux))) \|\| '') }})	0s
build sdist	11s
build-wasm-emscripten	2m 35s
coverage	2m 9s
lint	59s
test-debug 3.11	58s
test-debug pypy3.10	1m 22s
test on ubuntu	2m 7s
test-pydantic-integration	3m 0s
test on macos	4m 26s
test on windows	4m 11s
test 3.8	2m 5s
test 3.9	2m 18s
test 3.10	2m 5s
test 3.11	2m 24s
test 3.12	2m 27s
test pypy3.9	2m 40s
test pypy3.10	2m 36s
test build on ${{ matrix.target }}-${{ matrix.distro }}	0s
test build on ${{ matrix.os }}	0s
inspect-pypi-assets	0s
check	3s
release	0s
	41m 26s

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